This application is for continuation of a project aimed at investigating the mitogenic signal transduction mechanisms involved in hepatocyte proliferation late in gestation in the rat. The proposal is based on results of this project obtained during the last cycle which demonstrated that fetal rat hepatocyte proliferation in vitro and in vivo occurs without activation of a major mitogenic signaling pathway involving Mitogen Activated Protein Kinases (MAPKs). While this pathway was intact and could be activated by mitogens in cultured fetal rat hepatocytes, fetal hepatic MAPKs could not be activated in vivo by the intraperitoneal administration of Epidermal Growth Factor (EGF) to intact fetuses. This "uncoupling' occurred even though signal initiation by the EGF receptor tyrosine kinase was intact. In contrast to hepatic MAPK regulation, a parallel pathway involving Jun N-terminal Kinases (JNKs) was found to be active in fetal (but not adult) liver 171 vivo under basal conditions. Based on these findings, we propose the following specific aims: [1] To determine the mechanisms responsible for uncoupling of hepatic MAP kinase activation in the late gestation rat, and [2] To examine the significance of constitutive fetal hepatic JNK activity as it relates to regulation of gene expression. The overall strategy for these studies will be to carry out in vitro experiments using cultured fetal rat hepatocytes to identify potential mechanisms related to MAPK uncoupling and JNK-dependent control of growth-regulating genes. We will then carry out parallel comparative in vivo studies by examining the response to direct intraperitoneal injection of growth factors or insulin to the fetal rat. Key areas of emphasis will be the developmental aspects of the expression and function of constituents of hepatic mitogenic signaling pathways, JNK- dependent transcription factor (AP-I, ATF-2) regulation and JNK- dependent control of growth-regulating gene expression. We anticipate that our results will have implications for the physiology and pathophysiology of perinatal hepatocyte growth and hepatic development.